Chemotherapeutic anticancer agents are the drugs with the most restrictive therapeutic window. In fact, since their cytotoxic activity is non-selective they may indiscriminately damage all the cells of the body with which they come into contact.
There currently exists the problem of directing the cytotoxic agent selectively against the tumour cells, allowing the agent to exert its activity without damaging the cells of the healthy surrounding tissues, or at least limiting the damage as much as possible.
It has been reported in the literature that blocking the integrins αvβ3 and αvβ5 by means of the use of selective cyclopeptides, the reference compound for which is regarded as cyclopentapeptide c(Arg-Gly-Asp-D-Phe-Val) (JACS 1997, 119, 1328-35; international patent application WO 97/06791), or by means of the use of monoclonal antibodies (Cell, 1994, 79, 1157-64) leads to the blocking of angiogenesis and to a reduction of tumour growth. In addition, antimetastatic effects have also been observed (J. Clin. Invest., 1995, 96, 1815). Brooks et al. (Science, 1994, 264, 569-71) reported that the endo-thelial cells of the tumour vasculature and the tumour cells themselves preferentially express integrin αvβ3 compared to the quiescent cells of normal tissue. Among the compounds at an advanced stage of clinical development, we may mention c(Arg-Gly-Asp-D-Phe-MeVal), or EMD121974 or cilengitide.
Ruoslati and co-workers (Current Opinion in Oncology, 1998, 10, 560-5) showed that RGD analogues that bind to the tumour endothelium, once conjugated to the cytotoxic agent doxorubicin, form compounds that are more efficient and less toxic than doxorubicin alone. These authors also demonstrated, beyond any reasonable doubt, that the effect is attributable to the conjugation to RGD, inasmuch as the binding is antagonised by the free peptide itself (Arap, Pasqualini and Ruoslati, Science, 1998, 279, 377-380). Later, the same authors carried out other experiments consisting in chemically binding a pro-apoptotic peptide sequence to an RGD analogue, demonstrating that the new compounds were selectively toxic for angiogenic endothelial cells and had anticancer activity in mice (Ruoslati, Nature Medicine, 1999, 5, 1032-8).
Marcus et al., in international patent application WO 01/17563, describe specific anticancer activity for cytotoxic agents, such as camptothecin, conjugated by means of a spacer, consisting of one or more amino acids, to a non-peptidic inhibitor antagonist of integrins αvβ3 and αvβ5.
Aoki et al., Cancer Gene Therapy, 2001, 8, 783-787 describe the specific anticancer activity of a histidylated oligolysine conjugated to an RGD sequence, revealing a homing effect for tumours in mice.
The concept of binding at the cell surface mediated by integrins has been proposed for gene transport (Hart, et al., J. Biol. Chem., 1994, 269, 12468-12474).
7-tert-butoxyiminomethylcamptothecin (or CPT184 or ST 1481 or Gimatecan) is a derivative of camptothecin which is active orally and is described in European Patent EP 1 044 977.
It has now been found that 7-t-butoxyiminomethylcamptothecin conjugated in position 20, possibly by means of suitable spacers, to cyclopeptide derivatives containing the RGD sequence yields compounds endowed with high, selective anticancer activity which can be advantageously used for the preparation of medicaments for the treatment of tumours.
By virtue of their selective cytotoxic activity on tumour cells, the compounds according to the present invention yield medicaments with fewer and less severe side effects.